HR: 1340h
AN: OS13B-0528    [Abstracts]
TI: Argon Isotopes in Seawater: Fractionation During Air-Water Exchange and The $\delta^{40}$Ar of Seawater
AU: * Emerson, S R
EM: emerson@u.washington.edu
AF: School of Oceanography, University of Washington P.O. 355351, Seattle, WA 98195 United States
AU: Caillon, N
EM: caillon@lsce.cnrf-gif.fr
AF: Laboratoire des Sciences du Climat et de l'Environnement, CEA-CRNS Orme des Merisiers CEA Saclay, Gif Sur Yvette, 91191 France
AU: Hamme, R C
EM: rhamme@ucsd.edu
AF: Scripps Institution of Oceaongraphy, UCSD, Mail Code 0244, La Jolla, CA 92093 United States
AU: Severinghaus, J P
EM: jseveringhaus@ucsd.edu
AF: Scripps Institution of Oceaongraphy, UCSD, Mail Code 0244, La Jolla, CA 92093 United States
AB: We present a status report on a study of the argon 40/36 ratio in seawater. Results from laboratory experiments of the equilibrium and kinetic isotope fractionation factors during air-water exchange are used to interpret initial seawater measurements. Equilibration experiments in which fresh water was incubated with air at constant temperature and pressure resulted in $\delta^{40}$Ar values of 1.05 $\pm$ 0.01 $\permil$ (with respect to air at 25$\deg$ C) and 1.21 $\pm$ 0.01 $\permil$ (2$\deg$ C), which are about 0.4 $\permil$ greater than values for O$_{2}$ and N$_{2}$ but have the same temperature dependence. Kinetic fractionation factor measurements, in which the isotope ratio of pure argon was monitored in the head space of a reaction vessel containing initially gas-free, distilled water, indicate a fractionation factor of -5 $\permil$ during gas exchange - two to four times the values previously determined for O$_{2}$ and N$_{2}$. Initial measurements in the ocean at the Hawaii Ocean Time series indicate surface water values of 1.06 $\pm$ 0.02 $\permil$ (5 m and 23.5$\deg$ C) and 1.12 $\pm$ 0.01 $\permil$ (4000 m and 2$\deg$ C). Surface values are, within error, in equilibrium with the atmosphere, but deep values are 0.1 $\permil$ lighter than expected at equilibrium. It has been shown recently that Ar is undersaturated with respect to atmospheric equilibrium in the deep ocean by 1-2 %. This result is interpreted to be caused by a combination of processes that occur during deep water formation in high-latitude surface waters - rapid cooling leading to undersaturation and bubble formation causing supersaturation. One must know the importance of each mechanism to use the inert gas saturation state as a tracer for deep-water formation processes. Because of the large kinetic isotope fractionation factor, the argon isotope ratio is sensitive to diffusion across the air-water interface and may be used to separate these two mechanisms. We use a simple model to show that a deep water isotope ratio 0.1 $\permil$ lighter than the equilibrium value indicates that the thermally-driven degree of argon undersaturation must be about 50% greater than the measured value. To derive useful interpretation from these data one must be able to make the isotope ratio measurement to an accuracy of $\pm$ 0.01 $\permil$ because of the small differences between the deep water measurements and saturation values. Thus, this initial interpretation will have to be confirmed with more measurements and perhaps a better model.
DE: 4805 Biogeochemical cycles (1615)
DE: 4820 Gases
DE: 4894 Instruments and techniques
DE: 4227 Diurnal, seasonal, and annual cycles
DE: 4504 Air/sea interactions (0312)
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting